Throughout almost the whole series of living beings. however, we find concurrently with the process of agamagenesis, or a sexual generation. another method of generation, in which the development of the germ into an organism resembling the parent depends on an influence exerted by living matter differing from the germ. This is gamogenesis, or sexual generation. Looking at the facts broadly, and without reference to exceptions in detail, it may be said that there is an inverse relation between agamogenetic and gamogenetic reproduction. In the lowest organisms the latter has not yet been observed, while in the highest the former is absent. In many of the lower forms of life, agamo genesis is the common and predominant mode of reproduction, while munogenesis is exceptional; on the contrary, in many of the higher, while gamogenesis is the rule, agamogenesis is an occasional exception. In the simplest condition, that termed Coro jugation, sexualgeneration consists in the coalescence of two similar masses of proto plasmic matter, derived from different parts of the mine organisms of the same species, and the single mass which results from the fusion develops into a new organism. In the majority of cases, however, there is a marked morphological difference between the two factors in the process, and then one is called the male, and the other the female element. The female element is relatively large, and undergoes but little change of form. In all the higher plants and animals, it is a nucleated cell, to which It greater or less amount of nutritive material, the food-ycik, may be added. The male element, on the other hand, is relatively small. It may be conveyed to the female element by an outer growth of the wall of its cell, which is short in many alga and fungi, but becomes an immensely elongated tubular filament in the case of the pollen cell. of flowering plants. But more commonly the protoplasm of the male cell becomes converted into rods or filaments, which usually are in active vibratory movement, and sometimes are propelled by numerous cilia. Occasionally they are devoid of mobility, as in many artliropoda and n•natoidea. The manner in which the contents of the pollen tube affect the embryo cell in flowering plants is unknown, as no perforations through which the contents of the pollen tube may pass so as actually to mix with the substance of the embryo cell have been discerned; and there is the same difficulty with respect to the conjugative processes of some of the eryptogainia. But in the great majority of plants, and in all animals, there can be no doubt that the substance of the male element actually mixes with that of the female, so that in all these cases the sexual process remains one of conjugation; and impregnation is the physical admixture of protoplasmic matter derived from two sources, which may be different parts either of the same organism, or of different organisms.
The effect of impregnation appears in all cases to be that the impregnated protoplasm tends to divide into portions (b/astomere./), which may remain united as a single cell aggregate, or some or all of them may become separate organisms. A longer or shorter period of rest, in many cases, intervenes between the act of impregnation and the com mencement of the process of division. As a general rule, the female cell which directly receives the influence of the male, is that which undergoes division and eventual devel opment into independent germs; but there are some plants, such as the jtoridm, in which this is not the case. In these the protoplasmic body of the trichogyne, which unites with the molecular spermatozoids, does division itself, but transmits sonic influence to adjacent cells, in virtue of which they become subdivided into inde pendent germs or spores. There is still much obscurity respecting the reproductive pro cesses of the infusoria; but, in the torticellithr, it would appear that conjugation merely determines a condition of the whole organism, which gives rise to the division of the endoblast. or so-called nucleus, by which germs are thrown off; and if this be the case the process would have some analogy to what takes ,place in the fforidc6. On the other band, the process of conjugation by which two distinct diporper3 combine into that extra ordinary double organism, the diplozooa paradozant, does not directly give rise to germs, but determines the development of the sexual organs in each of the conjugated individ uals; and the same process takes place in a large number of the infusoria, if what are sup posed to be male sexual elements in them are really such. The process of impregna
tion in the A»•id4.0 is remarkably interesting from its bearing upon the changes which fecundation is known to produce upon parts of the parental organism, other than the ovum. even in the highest animals and plants.
The nature of the influence exerted by the male upon the female element is unknown. No morphological distinction can be drawn between those cells which are capable of reproducing the whole organism without impregnation, and those which need it, as is obvious from what happens in insects, where eggs which ordinarily require impregnation —exceptionally, us in many moths, or regularly, as in the case of drones among bees— develop without impregnation. In fact, generation may be regarded as a particular case of cell multiplication, and impregnation simply us one of the many conditions which may determine or affect that process. In the lowest organisms, the simple protoplasmic mass divides, and each part retains all the physiological properties of the whole, and conse quently constitutes a germ whence the whole body can be reproduced: In more advanced organisms each of the multitude of cells into which the embryo cell is con verted at first, probably retains all, or nearly all, the physiological' capabilities of the whole, and is capable of serving as a reproductive germ; but as division goes on. and many of the cells which result from division acquire special morphological and phy siological properties, it scents not improbable that they, in proportion, lose their more general characteristics. In proportion, for example, as the tendency of a given cell to become a muscle cell or a cartilage cell is more marked and definite, it is readily conceivable that its primitive capacity to reproduco the. whole organism should be reduced; though it might not be altogether abolished. If this view is well based, the power of reprouacing the whole organism would be limited to those cells which had acquired no special tendencies, and consequently had retained all the powers of the primitive cell in which the organism commenced its existence. The more extensively diffused such cells were, the more generally might multiplication by budding or fission take place; the more localized, the more limited would be the parts of the organism in which such a process would take place, and even where such cells occurred, their devel opment or non-development might be connected with the conditions of nutrition. It depends on the nutriment supplied to the female larva of a bee whether it shall become a neuter or a sexually perfect female; the sexual perfection of a large proportion of the internal parasites is similarly dependent on their food, and perhaps on other conditions, such as the :temperature of the medium in which they live. Thus the gradual disap pearance of agamogenesis in the higher animals would be related witlt that increasing specialization of function which is their essential characteristic; and when it quite ceases to occur, it may be supposed that no cells are left which retain unmodified the powers of the primitive embryo cell. The organism is then like a society in which every- one is so engrossed by his special business that he has neither time nor inclination to marry. Even the female elements in the highest organisms, little as they differ to all appearance from undifferentiated cells, and though they are directly derived from epithelial cells which have undergone very little modification from the condition of hlastomeres, are incapable of full development unless they are subjected to the influence of the male element, which may be compared to a kind of nutriment. But it is a living nutriment, in some respects comparable to that which would be supplied to ttn animal kept alive by transfusion, and its molecules may transfer to the impregnated embryo cell any special characters of the organism to which it belongs.